Steep III - Pacific Northwest Conservation Tillage Systems Information Source

PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 2 - Systems and Equipment, No. 14, Summer 1989

Legumes crops grown for seed or as a green manure can often help improve the production potential of the following cereal crop in the rotation. However, the traditional practice of plowing under a legume crop as a green manure before conventional seeding of a winter cereal can be disastrous where soil erosion is a potential problem. Very high rates of soil erosion can occur overwinter when the' residue from a green manure legume is completely buried and the winter cereal is planted into a fine smooth seedbed. Leaving a moderate amount of surface residue, with reduced tillage incorporation of green manure legumes, can bean effective option to control erosion while still achieving the other benefits of the legume crop.

Legume Rotation Benefits

There has been an increasing interest in legumes for seed, forage and as green manure in crop rotations in recent years. Nitrogen (N) fixation from N in the air can help to reduce N fertilizer inputs and production costs for the following crop. The amount of N fixation by the legume crop depends on the amount of plant-available N in the soil, legume species, growing conditions and other factors.

Besides potentially contributing some N, legumes often also have some other important benefits for the following cereal crop in rotation. These benefits are often lumped into what, is termed the "rotation effect". This is largely the result of reduced incidence of diseases and other pests associated with cereals or any other crop in the rotation. Other possible factors, such as increased activity of beneficial soil microbes, improved soil aggregation, reduced soil compaction and unused stored soil water available for the following crop, may also be part of the rotation effect. In addition to legumes, other non-cereal crops, such as rape seed, provide a rotation effect benefit for the following cereal crop.

Green Manure Tillage Research

The results of a 1985-86 field research study near Pullman, WA, demonstrate that a moderate amount of crop residue from green manure legumes can be maintained on the surface to control soil erosion without influencing the yield potential of the following winter wheat crop. The research was conducted by David Bezdicek, soil microbiologist at Washington State University, in cooperation with other researchers from WSU and the USDA-Agricultural Research Service. The research included a comparison of Austrian winter pea and red clover as green manure crops preceding winter wheat under conventional reduced tillage systems.

Tillage and Rotation Comparisons

Different crop rotations were utilized to allow inclusion of both red clover (a biennial crop) and Austrian winter pea (a fall-seeded annual) in the study. All of the research site was in spring barley in 1983. For the red clover trials, spring wheat was seeded in 1984 and immediately over seeded with red clover using a double disk drill. For the Austrian winter pea trials, winter wheat was seeded in the fall of 1983 and Austrian winter pea in the fall of 1984. Vigorous growth of both legumes was sufficient to control all weeds through the 1985 growing season. The conventional and reduced tillage treatments were conducted in July.

In both tillage treatments, the legume residue was flail chopped to facilitate ease of equipment operations and incorporation of the residue. Conventional tillage practices included moldboard plowing, followed by a shallow disking. The reduced tillage treatments were shallow disked twice. A non-selective herbicide was applied to all plots prior to seeding winter wheat. Both tillage treatments were seeded in October with a conventional double disk drill. The effects of N fertilizer rates of O, 60 and 120 pounds N/acre for the winter wheat were also compared,

A no-till winter wheat trial after a seed crop of spring dry pea was included in the study for comparison with the two green manure legumes. The primary reason for this treatment was to provide a comparison between forage/green manure legumes and seed legumes as crop rotation options.

Surface Residue

As seen in Table 1, the reduced tillage treatment left substantially more residue, of both green manure legumes, on the surface after seeding the winter wheat than with conventional plowing and disking. No difficulties were encountered in seeding through the residue with a conventional double disk drill.

Winter Wheat Yields

At each N fertilizer rate, winter wheat yields were slightly higher with reduced tillage than conventional tillage after the green manure legumes (Table 2). One exception was a slightly lower winter wheat yield under reduced tillage than conventional tillage following red clover at the O pound N/acre rate. The 60 pounds N/acre fertilizer rates substantially increased wheat yields after Austrian winter pea under both tillage systems and after red clover under reduced tillage. The 120 pounds N/acre rate generally gave little or no yield increase over the 60 pounds N/acre rate with the green manure legumes. With no N fertilizer, the yield of winter wheat after both green manure crops under both tillage systems compared favorably with yield of no-till winter wheat after a seed crop of spring dry peas.

Table 1. Estimated amount of surface residue and percent surface cover remaining after seeding winter wheat following green manure crops of Austrian winter pee and red clover under conventional and reduced tillage systems, 1985, Pullman, WA (Bezdicek and others, WSU).

Tillage System	Surface residue amount 1 and percent residue cover
Tillage System	Austrian winter pea		Red clove
	(lb/acre)	(% cover)	(lb/acre)	(% cover)
Conventional	130	8	80	5
Reduced	1,410	60	890	44

Table 2. Influence of conventional and reduced tillage systems and nitrogen (N) fertilizer rate on yield of winter wheat following Austrian winter pea and red clover green manure crops, 1986, Pullman, WA (Bezdicek and others, WSU).

Previous crop and N fertilizer rate	Tillage system
Previous crop and N fertilizer rate	Conventional	Reduced
Austrian winter pea
0	55	59
60	65	71
120	67	69
Red clover
0	75	70
60	73	78
120	78	82
Spring peas
0	-	59
60	-	61
120	-	59

Legume Residue N Recovery

One management concern with green manure legumes, and legumes in general, is the speed of release and availability of the plant residue N for the following crop. Will the N in residue remaining on the soil surface be as available as the N in residue completely incorporated in the soil? One approach Bezdicek and other researchers have used to help answer this question is through the use of N-15, a heavier' 'labeled" N source which can be differentiated from the normal N-14 available in the soil through laboratory analysis. This method can help track the movement of N from the plant residue.

The research included a combination of greenhouse and field experiments in 1987 and 1988. First, N-15 fertilizer was added to greenhouse pots of spring peas over a growing season to obtain a plant N-15 uptake of 20 percent or more. The plants were clipped at the green manure development stage and either soil incorporated or applied to the surface on small confined plots in the field. Winter wheat was seeded in the small plots and the level of N- 15 of the wheat plant and soil was monitored in the laboratory periodically throughout the growing season.

Based on the results of the study, uptake of the spring pea residue N by the wheat was essentially the same under soil-incorporated and surface residue treatments. Nearly 100 percent of the spring pea residue-N was accounted for in the wheat plant and soil analyses under the soil incorporate treatment. Recovery of residue-N from the wheat plant was similar under the residue placement treatments. Recovery of residue-N from the soil was about 10 percent (roughly 10 pounds N/acre in this case) less with the surface residue application than with soil incorporation. The mechanisms for this small amount of N losses with surface application of the pea residue are not known at this time. Bezdicek points out, however, that this small difference in N recovery from the soil probably would not result in a significant yield difference in following crops.

Chemical Kill and No-Till Seeding

The 1985-86 field study by Bezdicek also included a chemical kill/no-till seeding treatment after the two green manure legumes, along with the conventional and reduced tillage treatments previously discussed. Corresponding winter wheat yields tended to be lower than with either of the other two tillage treatments. So far, however, there have been too many unanswered questions about herbicide interactions and other possible factors to draw any firm conclusions from the results. A new field study near Pullman is currently underway to help answer some of the questions. Bezdicek and Alex Ogg, USDA-ARS agronomist, planted winter wheat in the fall of 1988 following several chemical and mechanical green manure treatments on Austrian winter peas. All of the plots were seeded with the new USDA Cross-Slot no-till research drill. The drill can effectively place seed and fertilizer in tilled and notill conditions using a unique single disk/knife opener arrangement. The green manure treatments include:

The soil fumigation treatment (No. 6) will help to determine whether soilborne diseases are associated with chemical-kill and/or no-till seedings. The study should help to separate the chemical, tillage, disease and residue factors affecting winter wheat yield potential following green manure legumes.

Conclusions

Based on the preliminary results of these studies, Bezdicek concludes that reduced tillage is a viable option for green manure legumes. He found that a moderate amount of green manure legume residue can be maintained on the soil surface past seeding of the following winter wheat crop without reducing wheat yield potential. This approach will help control erosion and still maintain availability of N from the legume residue.

The results of chemical-kill/no-till seeding of winter wheat after green manure legumes in the 1985-86 experiments were inconclusive. The new 1988-89 field study underway should help to provide more insight into the feasibility of this management option.